Presently, the interest is in particular directed to sensor assemblies, which are adapted to recognize a state of a roadway. Such a sensor assembly can for example include a sensor device, by means of which it can be recognized if the roadway is dry or if water, ice or snow is on the roadway. Based on the state of the roadway, the coefficient of friction between the roadway and the tires of the motor vehicle can be determined. This information can be used by a driver assistance system of the motor vehicle. For example, an information interface to the driver can be controlled thereby and/or the speed of the motor vehicle can be adapted depending on the recognized state of the roadway and the coefficient of friction, respectively. Such a driver assistance system can also be a traction control system or an electronic stability program, to which the information with respect to the state of the roadway is supplied.
In this context, DE 37 28 708 A1 describes a method for recognizing the coefficient of friction between a roadway and the tire of a motor vehicle. Herein, the rolling noises of at least one tire are captured by a microphone and the captured signal is analyzed in frequency. The roadway state can then be inferred from the frequency spectrum.
Moreover, DE 100 09 911 C1 describes a system for monitoring the air pressure of a motor vehicle tire by means of a sensor disposed in an air volume of an air spring. Due to the wheel oscillations, acoustic oscillations arise in the air volume, which are captured by the sensor. The sensor can be formed as a microphone or as a pressure sensor. Therein, the wheel oscillations are transmitted to the air spring via the chassis as structure-borne sound. Herein, the sensor can be formed as a microphone or as a pressure sensor.
Furthermore, from DE 10 2008 014 513 A1, a device for capturing a rolling noise of a vehicle tire is known. The device includes a sensor, which serves for capturing a rolling noise and which is disposed in a housing, which is fixed to the vehicle in oscillation-isolated manner. The sensor is preferably formed as a microphone. Alternatively, another suitable sensing element, for example a pressure sensor or an ultrasonic sensor, can also be employed as the sensor.
In order to early recognize the recognition of the occurrence of aquaplaning risk, DE 10 2010 008 258 A1 proposes a method, in which information related to course section with respect to the risk of aquaplaning is provided to the motor vehicle. In addition, at least one sensor device for determining a wet roadway is provided. Upon presence of a course section of the route with aquaplaning risk and the detection of a wet roadway, an assistance function for preventing aquaplaning is performed.
From DE 10 2012 221 518 A1, a method for ascertaining road slipperiness is additionally known. Herein, the spray projected by a wheel of the vehicle is sensorially captured. Road slipperiness is calculated from the projected spray. For capturing the projected spray, a sensor can for example be used, which is formed as a drop detector. Furthermore, a humidity sensor or a sensor for non-contact capture or counting of projected particles can be used. Water droplets, snowflakes, ice particles and/or dirt particles can be captured by the sensor.
Moreover, in the article “Sensor Systems and Signal Processing for Advanced Driver Assistance” by K. Naab and R. Hoppstock, published on the congress “Smart Vehicles”, 1995, a method is described, in which the water impacting on a wheel arch lining is captured by a sensor. Further, it is described there that the noise in the wheel case varies with the vehicle speed, the engine speed and the water level on the roadway surface. Therein, the first two factors can be easily corrected such that only the portion of the water impacting on the wheel arch lining remains. The noise can be measured by a microphone. Therein, the noise can be filtered by a band-pass filter in a frequency range between 2.5 and 4.5 kHz. Further, the noise can be converted to a characteristic direct current voltage level, which correlates with the absolute water level on the roadway surface.
It is the object of the present invention to demonstrate a solution, how a sensor assembly for recognizing a state of a roadway of the initially mentioned kind can be more inexpensively and robustly provided.